An organic compound can take various structures in comparison with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular design of an organic compound. Owing to those advantages, electronics utilizing a functional organic material has been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are given as examples of an electronic device utilizing an organic compound as a functional material. Those are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been remarkably developed.
It is said that the light emission mechanism of a light-emitting element is as follows: by application of voltage between a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined in the light-emitting layer to form molecular excitons and when the molecular excitons relax to a ground state, energy is released to emit light. Singlet excitation (S*) and triplet excitation (T*) are known as excited states. Light emission is considered possible through either singlet excitation or triplet excitation. Further, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
As for a compound in which a singlet excited state is converted to light emission (hereinafter, such a compound is referred to as a “fluorescent compound”), light emission from a triplet excited state (phosphorescence) is not observed but only light emission from a singlet excited state (fluorescence) is observed at a room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on the relationship of S*:T*=1:3.
On the other hand, when a compound in which a triplet excited state is converted into light emission (hereinafter, such a compound is referred to as a “phosphorescent compound”) is used, internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency that is 3 times to 4 times as much as that of the fluorescence compound can be achieved. For those reasons, in order to achieve a highly efficient light-emitting element, a light-emitting element in which a phosphorescent compound is used has been actively developed recently.
When a light-emitting layer of a light-emitting element is formed using the above phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation (T-T annihilation), the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix of another substance. In the above case, the substance, which serves as a matrix, is referred to as a host material, and the substance, like a phosphorescent substance, which is dispersed in a matrix, is referred to as a guest material.
In the case where a phosphorescent compound is used as a guest material, a host material is required to have a large energy gap (a difference between the highest occupied molecular orbital level (HOMO level) and the lowest unoccupied molecular orbital level (LUMO level)) or higher triplet excitation energy (a difference in energy between a ground state and a triplet excited state) than that of the phosphorescent compound. Therefore, a substance having such characteristics has been developed.
For example, in Non Patent Document 1, a material which has a quaterphenylene skeleton is used as a host material of a phosphorescent compound which exhibits blue light emission and as a hole-transporting layer.
[Non Patent Document 1]
    J. Kido et. al., Chemistry Letters, Vol. 36, No. 2, 316-317 (2007)